Trailer weight 1971 C20 documentation

[68 50th]

I have a 1971 C20 originally equipped with a 402 bigblock and 4.10 Dana axle. I recently put on a trailer hitch and need to have that registrered to the truck. Over here in Sweden the truck will get 1500kg (3307lb) as a maximum trailer weight without any documents. But if I could show them a GM document or other that says something higher I will probably get that higher number in the registration. The paperwork for the trailer hitch itself says 3000kg (6614lb) but that doesn´t help if I can not show them trailer weight capacity documents for the truck.

So does anyone have this kind of document or can point me in the right direction to find it?

Thanks in advance.